The present invention relates generally to a breathing apparatus or ventilator and method for supplying inspiratory gases to a living being such as a human or animal patient, and is particularly concerned with relieving the breathless sensation often encountered by individuals on ventilators.
A breathing apparatus or ventilator is generally connected to a patient through a patient circuit having an inspiratory limb and an expiratory limb. The ventilator delivers gases to the patient from a gas delivery unit through the inspiratory limb during inspiration, and allows removal of expired gases through the expiratory limb during expiration. Pressure targeted ventilation is a type of ventilation in which the ventilator tries to reach a pre-set pressure level and maintain that level during the inspiration phase. There are two types of pressure targeted ventilation, called pressure control ventilation and pressure support ventilation, or PSV.
One problem with all types of ventilation is that patients sometimes feel breathless even though the work of breathing has been reduced significantly. This is because the amount of gas supplied by the ventilator at a certain time may not necessarily correspond to the patient""s own effort to inhale, or the patient inspiratory muscle pressure change (Pmus). From the physiological standpoint, when the patient generates a certain level of Pmus, he would expect a certain level of flow into his airway. For the patient under mechanical ventilation, if the flow provided by the ventilator is much lower than that level at a given Pmus, the patient will feel breathless (or encounter a resistive load detection). This principle is described in a paper entitled xe2x80x9cEffect of timing, flow, lung volume, and threshold pressures on resistive load detectionxe2x80x9d by Killian et al., Journal of Applied Physiology 1980; 49:958-963. It has also been found that the breathless sensation occurs more at the early phase of the inspiration than the late phase. In patients who have respiratory failure due to high airway resistance, additional resistance from the endotracheal tube and patient circuit, and/or reduced respiratory compliance, the flow as a function of patient Pmus falls below the threshold. In order to compensate for this, patients will increase their inspiratory effort, or Pmus, causing them to feel even more breathless.
The breathless sensation is a problem even in pressure targeted ventilation, since the patient effort is not taken into account in the control system of such ventilatory modes. The control system simply aims to maintain the set pressure level, or Paw, during the inspiratory phase, resulting in a quasi square pressure waveform.
Some other types of ventilation aim to take patient effort into account to some degree. Younes introduced proportional assist ventilation in his article xe2x80x9cProportional assist ventilation, a new approach to ventilatory supportxe2x80x9d, American Review of Respiratory Diseases 1992;145(1):114-120, U.S. Pat. No. 5,044,362. During proportional assist ventilation, the ventilator is controlled in such a way that the pressure delivered at the airway increases in proportion to the patient spontaneous effort throughout the whole inspiration. The delivered pressure is controlled by two factors, flow assist (resistive gain) and volume assist (elastance gain).
Under proportional assist ventilation, the patient has a very high level of freedom and capability of controlling the ventilator, which can cause problems. For many patients in intensive care units, too much freedom may mean underventilation if the patient""s inspiratory effort becomes weak, or overventilation if the patient""s inspiratory effort becomes aggressive. Also, in proportional assist ventilation the ventilator control system may xe2x80x9crun awayxe2x80x9d if volume assist is set below patient elastance. The ventilatory support during proportional assist ventilation is proportional to the patient muscle pressure throughout the whole inspiration. Therefore, in proportional assist ventilation, airway resistance and respiratory compliance values representative for the whole inspiratory phase must be accurately calculated for the purposes of accurate ventilator control and ventilatory management.
In the Drager Evita 4 ventilator, as described in the xe2x80x9cDrager Evita 4 Operating Manual (Drager Medizintechnik GmbH, Lubeck Germany) there is a breath mode called xe2x80x9cautomatic tube compensationxe2x80x9d. When this mode is used, the user needs to set the endotracheal tube size (resistance factor) and the percentage of tube compensation. The ventilator will then try to overcome the resistance imposed by the endotracheal tube by adding more pressure than the set value, with the use of the anticipated endotracheal tube resistance. This does not take into account any information of patient effort in the adjustment of the compensation levels. The compensation level is fixed and is solely determined by the user-set endotracheal tube size and percentage of tube compensation.
It is an object of the present invention to provide a new and improved pressure targeted breathing apparatus and method which relieves the breathless sensation sometimes encountered by patients on ventilators.
According to one aspect of the present invention, a breathing apparatus for providing positive pressure assistance is provided, which comprises a source of breathing gas, an inspiration line for connecting the source to a patient during an inspiratory phase, an expiratory line for exhausting gases from the patient during an expiratory phase, a pressure sensor for sensing pressure in the system, and a control unit for controlling supply of gas to a patient in each inspiratory phase according to a pre-set target pressure, the control unit being programmed to determine the level of patient breathlessness at periodic intervals and to calculate a boost pressure above the pre-set target pressure based on the determined breathlessness level, and to boost the pressure (and accordingly the flow) of gas supplied to the patient at the start of each inspiratory phase to the most recently calculated boost pressure in order to reduce patient breathlessness, and reduce the pressure back to the target pressure at a predetermined time after the start of the inspiratory phase and prior to the end of the inspiratory phase.
The boost pressure level may be determined by a user-adjustable controller on the apparatus to a selected breathless sensation assist level, or BSA, and may also be varied in proportion to the detected occlusion pressure at the pressure sensor, or Paw, at a predetermined time after the start of inspiration with the patient airway being temporarily occluded, for example at 0.1 seconds after the onset of inspiration (Paw-0.1). Thus, for any selected BSA greater than zero, the pressure boost level will be varied in proportion to the BSA level and detected Paw-0.1 for a preceding measurement or preceding measurements, and will increase with increase in Paw-0.1. This is because research on the relationship between the airway occlusion pressure or Paw and the patient inspiratory effort, or Pmus, have indicated that there is a good consistency between the two pressures at the beginning of inspiration. (Conti, G. et al., American Journal of Respiration and Critical Care Medicine, 1996, 154:907-912). Thus, boosting of pressure (i.e., flow delivery) in proportion to Paw-0.1 can relieve the breathless sensation of ventilated patients.
In one exemplary embodiment of the invention, the pressure boost magnitude is gradually tapered down from the boost level back to the target pressure level at around the middle of the inspiration phase of the respiratory cycle. The control unit is set up to measure the Paw-0.1 value at periodic intervals, and can be measured by delaying the onset of the inspiratory gas delivery for a predetermined time after the ventilator is triggered, such as 0.1 seconds. This allows for a reliable Paw-0.1 measurement without causing significant patient awareness or discomfort. The measurement may be made at predetermined intervals, for example every 20 breaths or every 2 minutes, with appropriate readjustment of the pressure boost level after every measurement.
Instead of boosting the pressure of gas supplied, gas flow to the patient may instead be boosted directly by a corresponding amount.
According to another aspect of the present invention, a method of controlling the inspiratory phase in a pressure targeted ventilation system is provided, which comprises the steps of setting a target pressure level for the inspiratory phase, determining a boost pressure level higher than the pre-set target pressure level, periodically determining the level of patient breathlessness and adjusting the boost pressure in response to changes in the level of patient breathlessness, boosting the pressure supplied to the patient to the most recently calculated boost pressure level at the start of the inspiratory phase in order to relieve any breathless sensation encountered by a patient, and gradually reducing the pressure from the boost level back to the pre-set target level at a predetermined point in the inspiratory phase.
The step of determining a boost pressure level may be carried out at predetermined time intervals or for each nth breath, for example at 2 minute intervals or every 20 breaths, although other time or breath intervals may be selected. The boost pressure level may be determined based on two factors, one of which is a user selected breathless sensation assist (BSA) level, and the other of which is based on a detected patient airway occlusion pressure or Paw-t at a short time interval of t seconds after the onset of a patient inspiratory effort. In one example, Paw-t was measured by delaying the onset of inspiratory gas delivery for a predetermined time, such as 0.1 seconds, after the ventilator was triggered, and analyzing the pressure detected by the pressure sensor for this time period, using the most linear segment of the slope of pressure vs. time to determine Paw-t, or Paw-0.1 where the time interval is selected to be 0.1 seconds.
This invention uses the patient""s physiological information, as determined by the periodic measurement of Paw-0.1, as well as a user adjustable BSA level, in order to boost the pressure of gas supplied to the patient to a predetermined level at the start of each breath or inspiration. This will overcome or reduce any breathless sensation which may otherwise be encountered by the patient if their inspiratory muscle pressure change does not result in an expected airway flow, as may be the case when a set and unchanging pressure level is used throughout the inspiratory phase.